Sedimentation and the Design of Settling Tanks

Sedimentation has been us ed for generations to clarify liquors and to concentrate solids in many widely diversified fields. It is the most commonly used process in the field of water and sewage treatment. The investments for settling tanks in this field are probably about one third of the total capital investment for treatment. Despite the importance of the process, current understanding of the principles involved has progressed so slightly that there is as yet no such thing in practice as the economic design of tanks from a functional viewpoint. The dimensions of most settling tanks are fixed on the basis of standard detention periods recommended by state health departments. This practice prevails in disregard of the fact that evidence was presented as early as 1904 that removal of suspended matter depends upon surface area and not upon tank volume. The settling characteristics of the suspensions to be clarified are rarely consid ered in the design of settling tanks.